Generally a piezoelectric material is used for an ultrasound transducer (ultrasonic transducer), since the piezoelectric material has a function to transform kinetic energy into electric energy or vice versa, what is called as a binding action between electrical system and mechanical system. Used piezoelectric material has the shape of a sheet, tabular or cylindrical having a pair of electrode in which one electrode is feed to a back layer, and another electrode contacts to medium through an acoustic lens or a matching layer.
Most of piezoelectric ultrasound transducers emit a sound wave to a medium by d33 mode or e33 mode, or detect the sound wave which propagates to medium d33 mode is generally said to be as a longitudinal oscillation of a pillar-shaped transducer, and e33 mode is as thickness vibration of a plate type transducer. In ferroelectrics, such as PZT (lead zirconate titanate) ceramics and PVDF (polyvinylidene fluoride), high-dielectrics such as P(VDCN/VAc) (vinylidene cyanide-vinylacetate copolymer), and a porous polymer electret piezoelectric material, remanent polarization is kept according to the orientation of the electric dipole by polling processing, and d33 and e33 are shown. On the other hand, with respect to a piezoelectric crystal without remanent polarization, C axis in the case of a piezoelectric crystal such as ZnO (zinc oxide), LiNbO3 (lithium niobate single crystal) and KNbO3 (potassium niobate single crystal), and A axis in the case of rock crystal is perpendicularly orientated against an electrode surface respectively, whereby d33 or e33 (in case of rock crystal, d11 or e11) are shown. In the case of a piezoelectric composite material, it depends on a used material.
Here, in the piezoelectric material which constitutes an ultrasound transducer, the simplest dynamic boundary condition is a case where one end is a fixed end and another end is a free end. Theoretically, there is a relationship between an acoustic impedance Z (unit is MRayl.) of a touched body and a boundary condition that Z=0 for the free end and Z=∞ for the fixed end. However, in the present specification, it is not so strictly defined as such. When an impedance Z of a piezoelectric material except for an adhesive layer and an electrode layer is small or equal to an impedance Z of a touched body, it shall be regarded as a fixed end, and when large, it shall be regarded as a free end. Moreover, a resonance of the longitudinal oscillation or thickness vibration of a piezoelectric material is used for wave transmission and wave reception of art ultrasound transducer. The resonance frequency fr is mainly decided by physical properties and dimension of a piezoelectric material, although it depends on a structure of a transducer or how to press against a medium. Therefore, in this specification, factors are eliminated which are other than the physical properties or the dimension of a piezoelectric material and affect change in resonance frequency.
The resonance frequency fr in d33 mode or e33 mode of a piezoelectric material is represented by the following Expression (1) from the sound velocity v of a piezoelectric material and height (thickness) h.fr=v/4h  (1)
Generally this is called λ/4 resonance. λ means a wavelength inside of the piezoelectric material. In addition, there is λ/2 resonance in which both ends are freed. The resonance frequency of λ/2 resonance is as twice as that of λ/4 resonance.
On the other hand, the sound velocity v of the above-mentioned piezoelectric material is represented by the following Expression (2) for the longitudinal oscillation of a pillar-shaped transducer, and the following Expression (3) for the thickness vibration of the plate type transducer. Herein, s represents elastic compliance, c represents elastic stiffness and ρ represents density.v=(1/sρ)1/2  (2).v=(c/ρ)1/2  (3)
It is understood from above-mentioned Expressions (1)-(3) that the transmission frequency and reception frequency of a transducer are mainly determined with height (thickness) h, elastic modulus s and density ρ of piezoelectric material.
It is required for the ultrasound diagnostic imaging apparatus used for a medical field to perform a high frequency of a transducer or to improve in a wave transmission-and-reception performance in order to obtain an image having a high resolution. In order to improve the wave transmission-and-reception performance in the ultrasound transducer using a piezoelectric material, it is an important factor for transmitting an electrical signal by a high S/N ratio to match electric impedances between a transducer and an electric processing circuit. Moreover, since a transmission-and-reception frequency is determined by a thickness of the piezoelectric material, it is necessary to make the piezoelectric material thinner for perforating a high frequency. A thinner piezoelectric material contributes to lower electrical impedance and to have, an advantage to an impedance matching with an electric circuit. However, the range of the reduction of this electrical impedance is at most an inverse of a thickness ratio. Moreover, the thinner piezoelectric material makes a manufacturing process difficult such as thickness control, or handling.
In the conventional technology, it is used a higher resonance component in the transmission-and-reception wave signal of the conventional λ/4 resonance transducer in order to obtain a high frequency signal. However, sensitivity of higher resonance component is weaker compared with a primary resonance component, and is easy to decrease by dumping of a piezoelectric material or circumference material, whereby there is problem that it is hardly to obtain the signal with a high S/N ratio. Here, with reference to FIG. 1, e33 thickness stretch mode will be explained as an example of the transmission-and-reception wave of the ultrasound using higher resonance mode. This FIG. 1 and the following explanation are shown in the Non-patent Document 1. Constants of the elements which constitute the equivalent circuit of this FIG. 1 are as follows:Cn=pnkt2C0  (4)L=1/ωpi2C1  (5)pn=(1/n2)(8/π2), n=2m−1  (6),
wherein Cn represents a capacitance of each element, L represents an inductance, kt represents an electromechanical coupling coefficient in thickness stretch mode and ωpi represents a resonance frequency.
When approximated with pn≈1/n2 in the above-mentioned Expression (6), Expression (4) will be:Cn/C0=kt2/n2  (7).
Expression (7) shows that the effective value of the electromechanical coupling coefficient in the n-th higher resonance mode decreases to 1/n. Since it is n=1 in the case of the primary resonance-mode, Expression (7) will be:Cn=1/C0=kt2  (8).
This Expression coincides with Expression (9) showing a relational expression of kt and dielectric constant in the primary resonance mode:∈T/∈S=1+kt2  (9),
wherein ∈T and ∈S are set as ∈T=C0+Cn, ∈S=C0; and ∈S represents a dielectric constant of bound conditions, ∈T represents a dielectric constant of free conditions, and C0 and Cn each represents electric capacitance. To d33 mode, the above-mentioned Expression (4) is replaced to the following Expression:Cn=pn(k332/l−k332)C0  (10),and the same result will be obtained.
When the 3rd resonance component is transmitted and received, an effective value of the electromechanical coupling coefficient is given by Expression (7) using n=3. When an apparent coupling coefficient is set with kt′, kt′=kt/n=kt/3. This result means that the apparent coupling coefficient declines to ⅓, when the 3rd resonance component is transmitted and received.
FIG. 2 is the graph which shows the frequency characteristic (calculated value) of the complex dielectric constant of the piezoelectric material which shows the primary resonance mode of thickness resonance at 1 MHz. Herein, kt=0.3, h/2v=2.485×10−7 (s) and tan δm=0.04.
The maximum and the minimum of the real part (referred to as a referential mark α1) and the maximum of an imaginary part (referred to as a referential mark α2) shown at 1 MHz are based on the primary resonance component of thickness resonance. Further, the 3rd resonance component is shown at 3 MHz, and the 5th resonance component at 5 MHz. On the other hand, when the 3rd resonance component shown in FIG. 2 is applied to a piezoelectric material model which has the primary resonance component at 3 MHz, it coincides with a case in which a coupling coefficient and thickness of the piezoelectric material are set to ⅓ as shown in FIG. 3. These results are in agreement with the above-mentioned interpretation. FIG. 3 is a graph which shows the frequency characteristic (calculated value) of the complex dielectric constant of the piezoelectric material which shows a thickness resonance. Herein, a dashed line is the case of kt=0.3, h/2v=2.485×10−7 (s) and tan δm=0.04 as mentioned above. On the other hand, a solid line is a case of kt=0.1, h/2v=8.300×10−7 (s) and tan δm=0.04.
As mentioned above, the problems of the conventional technology are that an apparent electromechanical coupling coefficient decreases to 1/n when detects n-th resonance component, and that an electrical impedance is unambiguously decided by the dimension of a piezoelectric material.
On the other hand, in an ultrasound diagnostic apparatus for medical application, Tissue Harmonic Imaging (THI) diagnosis using a harmonic signal occurred in a living body is becoming a standard diagnostic modality in view of obtaining a clear diagnostic image which can not be obtained by the conventional B mode diagnosis. When the used frequency becomes high such as Harmonic Imaging, there are many advantages such that a side lobe level becomes small, S/N and a contrast resolution becomes good, and a beam width becomes thin and azimuthal resolution becomes good, and since sound pressure is still smaller at close range and change of sound pressure is little, whereby multiple reflection does not happen.
In Patent Document 1, proposed is an ultrasound diagnostic apparatus in which a signal received by each piezoelectric element of the ultrasound transducer is added by the phasing summing circuit, input commonly to a filter of a fundamental-wave band, and a filter of a harmonics band, weighted to those outputs by gain respectively corresponding to the depth of the diagnosing area of a test object, and compounded, whereby attenuation of the harmonic component in a deep diagnosing area is interpolated by the fundamental wave. That is, in reception of harmonics, the fall of the above-mentioned electromechanical coupling coefficient is compensated by using a filter and an amplifier.
Similarly, in Patent Document 2, proposed is an ultrasound diagnostic imaging apparatus in which the piezoelectric element for harmonics is laminated to the piezoelectric element for the fundamental waves, and an ultrasound for transmission is emitted from the piezoelectric element with the fundamental frequency. On a signal component received by the piezoelectric element for these fundamental wave, a plurality of harmonic components received by the piezoelectric elements for harmonics and a desired component extracted by passing bandpass filter respectively are summed by adjusting gain individually, whereby the ultrasound diagnostic imaging apparatus which acquired the signal according to the depth of the diagnosing area.